xfrm: fix rcu usage in xfrm_get_type_offload

request_module can sleep, thus we cannot hold rcu_read_lock() while
calling it. The function also jumps back and takes rcu_read_lock()
again (in xfrm_state_get_afinfo()), resulting in an imbalance.

This codepath is triggered whenever a new offloaded state is created.

Fixes: ffdb5211 ("xfrm: Auto-load xfrm offload modules")
Reported-by: syzbot+ca425f44816d749e8eb49755567a75ee48cf4a30@syzkaller.appspotmail.com
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_state.c

@@ -313,13 +313,14 @@ xfrm_get_type_offload(u8 proto, unsigned short family, bool try_load)
 	if ((type && !try_module_get(type->owner)))
 		type = NULL;
 
+	rcu_read_unlock();
+
 	if (!type && try_load) {
 		request_module("xfrm-offload-%d-%d", family, proto);
 		try_load = 0;
 		goto retry;
 	}
 
-	rcu_read_unlock();
 	return type;
 }